Acid rain is caused by pollution in the atmosphere bonding with water droplets. The pollution itself is normally sulphur or nitrogen dioxide. These compounds combine with water in the atmosphere, fall as acid rain and enter the general water supply. Initially it is more harmful to plants and animals, but this rain can have devastating effects on older buildings and statues made from certain types of stone.
Older statues, especially those made of marble and limestone, are especially vulnerable to acid rain. This is because the rock contains large amounts of calcium carbonate, which reacts with the acids in the rain. This can cause the stone to turn to gypsum and flake off, ruining the detailing. This effect is cleaned off many statues, but can be regularly seen in graveyards where the headstones show the same erosion effect. Bronze statues are also at risk as the acid oxidises in the metal causing corrosion.
Due to what they are made of, buildings are less affected by acid rain. Older buildings with limestone or marble modelling on the outside can suffer from the same problem as statues. In addition, metalwork within the structure, such as copper pipes and wiring, can oxidise if exposed to acid rain.
To see the effects of acid rain on buildings and monuments, place two pieces of chalk in separate bowls. Fill one bowl with tap water and the other with white vinegar. The next day look at the pieces of chalk. The piece in the vinegar shows much more erosion. The chalk in this experiment bears many of the same characteristics as marble or limestone and the vinegar replicates the acidity of the rain.
Various national and international treaties have reduced the threat of acid rain. These have reduced the quantity of acid rain and modern techniques have allowed buildings and statues to be better protected. Some areas take more drastic measures by covering statues with tarpaulins during periods of the year when there might be acid rain.